1. Field of Invention
The present invention relates to an electronic circuit for an ion sensor, and more particularly to an electronic circuit for hydrogen ion sensitive transistors, configured as a bridge-type floating source with body effect reduction.
2. Related Art
The development of micro-electronic technology has enabled a very small semiconductor to respond to chemical concentrations. Ion-sensitive field effect transistor (ISFET), a kind of micro-sensing device invented by Bergveld in 1970 and developed quickly thereafter, is a solid state element consisting of a chemical sensing film and a field effect transistor. The ISFET, capable of selective measurements of concentration (activity) of certain ions in the electrolyte, is an impedance converting element, which combines the characteristics of the low output impedance of the metal oxide semiconductor field effect transistor (MOSFET) and the operation principle of the ion selective electrode (ISE). The ISFET has the advantages of short response time, batch processing ability, small sample volume, potential of single chip circuit integration, and compatibility to CMOS technology. Therefore, it is the best choice for the chemical sensor array in a large scale integrated circuit.
Comparing with the MOSFET element, in the ISFET structure, the metal or the poly silicon gate is replaced by an electrolyte and a reference electrode. Variations in the concentration of certain ions in the electrolyte result in the changes of the threshold voltage (dVTH*) of the ISFET. The changes reflecting the ion concentration in the electrolyte are recorded by a suitable electronic read-out circuit.
The manufacturing method, analysis of characteristics, and measurements of the ISFET have been widely discussed and studied. The research shows that the ISFET has some undesirable characteristics, including temperature dependence, time drift, and hysteresis, which limit the precision of measurements carried out by means of ISFETs. Furthermore, the slope of the characteristic (sensitivity) is also an important parameter describing the performance of ISFET.
The threshold voltage of ISFET will change due to the change in concentration of hydrogen ions (expressed by the pH change of the electrolyte), so that the voltage response is often used as an output signal of the ISFET. To capture the electrical signal generated by a sensor, the sensor (ISFET) must be accompanied with an analog read-out interface. The most precise measuring method by means of ISFETs is obtained under constant voltage/constant current conditions. Recently, along with the development of semiconductor technology, integration of an ISFET and a read-out interface into a chip has become an important research subject. Due to low migration and high carrier mobility, the N-channel ISFET is more often used than the P-channel ISFET. At present, in the CMOS technology, an NMOS is usually manufactured in the p-type body, and in order to make the circuit operate in a normal mode, the p-type body should usually be connected to the most negative voltage of the system. Therefore, if the ISFET and the read-out interface are integrated into the same chip, the above-mentioned read-out circuit will face the problem that the substrate potential seriously influences the real characteristics of the elements. Morgenshtein et al. proposed a new technology in 2004, which eliminates the body effect of ISFET in the read-out circuit of the integrated micro-system; however, some part of the circuit has not yet been applied to the constant voltage and constant current configuration.
Furthermore, many articles regarding compensating circuits have been published with respect to the ISFET temperature effect and time drift. Wang et al. proposed the method of zero temperature coefficient regulation and temperature coefficient compensation for reducing the temperature coefficient of the pH-ISFET. However, zero temperature coefficient regulation is more suitable for the time-controlled measurements for solutions of a specified pH range. For different ISFETs, the drain current uninfluenced by the temperature should first be determined. To compensate for the temperature and time drifts, Palan et al. applied the difference of sensitivity of ISFETs with two different sensing films (Si3N4 and Al2O3). The ISFETs were operated in a differential mode. However, there was not enough data to prove the compensating effect for this approach. Casans et al. proposed the regulating and compensating circuit controlled by voltage to eliminate the effects of time and temperature drifts. However, in this case many additional bias voltages must be provided, which increase the complexity of the system.
Some American patents have proposed methods for time and temperature drift compensation, but all those methods required additional circuits and redundant data calculation. Considering U.S. Pat. No. 4,641,249, the temperature drift compensation method required extra temperature sensors, and signal processing circuits, which adopted zero temperature regulation to calculate temperature compensation parameters. These resulted in large hardware circuits including processors, analog digital converters, digital-analog converters, random access memories, as well as read only memories, and complex calculations to be done on a computer. The time drift compensation method disclosed in the U.S. Pat. No. 4,701,253, includes ISFETs, amplifiers, and control compensating circuits, which adopts the index equation ΔVp=A ln(t/t0+1) to correct the time drift. Likewise, the circuit disclosed in the U.S. Pat. No. 4,701,253 also requires redundant calculation and data storage.
The issued methods use the constant voltage/constant current and the readout circuit configuration with floating reference electrodes to measure the ion concentration of a solution. As an example, let us consider the readout circuit configuration with grounded reference electrode and bridge-type floating source in FIG. 1. This circuit includes a bridge-type configuration consisting a current source Iref, a constant voltage source (where voltage is adjusted by a potentiometer Rv), and an operational amplifier OP, wherein the zener diode ZN provides a reference voltage of a specific voltage value, while the amplifier OP, resistors Ra, Rb, and Rc and the ISFET form a bridge network. Because of the balance of the bridge-type architecture and the virtual short circuit of the input terminals of the operational amplifier, the constant voltage VDS of the ISFET will be generated; then the constant current IDS passing through the ISFET will be determined by the resistor Rb. The advantage of this architecture is that the reference electrode is grounded so that only one common reference electrode is required to perform the multiple and simultaneous detection of ions by means of ISFETs. Since the Zener diode ZN with floating potentials at both terminals is fabricated in a special technological process, processing of this circuit is not compatible with a standard CMOS technology.